FIGS. 5(a)-5(c) illustrate a method of manufacturing a conventional long wavelength semiconductor laser. As illustrated in FIG. 5(a), there are successively grown on a p-type InP substrate 1 by a method, such as metal organic chemical vapor deposition (MOCVD), a p-type InP cladding layer 2, an undoped InGaAsP active layer 3, and an n-type InP cladding layer 4. The three grown layers form a conventional double heterojunction structure.
As shown in FIG. 5(b), a mask 5 of an electrically insulating film, such as silicon dioxide, is formed on a part of the n-type InP cladding layer 4. The mask 5 is used as an etching mask and the layers 2, 3, and 4 are etched to form a mesa or a ridge. After the formation of the ridge, with the mask 5 still in place, a p-type InP layer 6, an n-type InP layer 7, and a p-type InP layer 8 are successively grown by MOCVD. These layers do not grow on the mask 5 and the resulting structure is as illustrated in FIG. 5(b). The p-n-p-type layers 6, 7, and 8 are a current blocking structure that helps confine current flow between the n-type cladding layer 4 and the p-type cladding layer 2 to the ridge including the undoped active layer 3. The p-type layer 6 contacts the side surfaces of the n-type cladding layer 4, forming a pn junction that blocks current flow other than through the active layer 3.
A completed semiconductor laser is illustrated in FIG. 5(c). The mask 5 is removed and thereafter an n-type InP contact layer 9 is grown by MOCVD on the n-type InP cladding layer 4 and the p-type InP 8, forming a rectifying junction with that p-type InP layer 8. The structure is completed by forming electrodes 10 and 11 on the substrate 1 and the n-type InP contact layer 9, respectively.
Conventionally, in producing the semiconductor laser illustrated in FIG. 5(c), a source of zinc as a dopant impurity is present when growing p-type InP layers and a source of sulfur is present as an n-type dopant impurity when growing n-type layers. Although the gaseous source of zinc is absent when the InGaAsP active layer 3 is grown, the zinc incorporated in the p-type cladding layer 2 can diffuse into the active layer 3 during the growth of the active layer 3, the n-type cladding layer 4, and the layers 6-9, since MOCVD is carried out at an elevated temperature.
To reduce the series resistance of the semiconductor laser, it is desirable to incorporate as much zinc as possible in the p-type InP cladding layer 2. However, as the quantity of zinc dopant impurity in that layer is increased, the quantity of zinc that diffuses into the undoped InGaAsP active layer 3 increases substantially. The presence of zinc in the undoped InGaAsP semiconductor material significantly impairs the optical characteristics of that material and has adverse effects on the characteristics of the semiconductor laser. Thus, it has been impossible to reduce the resistance of the laser by reducing the resistivity of the p-type InP cladding layer 2 without causing significant adverse consequences to the optical performance of the semiconductor laser.